The present invention relates generally to the refining of cellulosic pulp, and more particularly, to a dry refining process and apparatus wherein the liberated fibers are structurally deformed and twisted without producing knots or fiber bundles.
Cellulosic fibers must be subjected to mechanical treatment before they can be made into paper. This treatment may be applied in a number of different ways, but it generally includes a bruising, rubbing or crushing action on the fibers. The terms beating and refining are used in the paper industry to describe the operation of mechanically treating pulp fibers. Refining usually refers to a fiber separation and fiber cutting action, whereas beating action may include these two effects, and also a fibrillating or bruising effect on the fiber. The amount and type of mechanical treatment used on the pulp contributes to the final pulp properties including burst, tensile strength, density, bulk, freeness and formation. In this regard, dry refining with a disk refiner is alleged to be effective treatment for enhancing some of the above noted properties of pulp. Dry refining may be defined as subjecting pulp to a refining action at a solids content in excess of about 85%.
In disk refining, the pulp generally enters the refiner housing through an opening near the center of one disk, passes into a central feeding chamber, and then passes outward between the disks where it is discharged at the periphery of the housing. Pulp throughput is controlled by the amount of material introduced into the refiner while the gap or space between the disk plates determines the amount of work or energy expended on the pulp. After refining, the pulp falls by gravity through an exit at the periphery of the refiner. The units may be of the single rotation type (with one stationary and one rotating disk), the double rotation type (with two rotating disks), or of the double disk type (two stationary disks and one double sided rotating disk). The distance between the disks is generally greater near the central part and successively decreases outwards into a narrow gap between the disk plates which have generally parallel sides along the outer parts of the disks. Pulp is conducted by centrifugal forces from the feeding chamber outwards through the gap between the disk plates where the pulp is subjected to the refining action. Thus, as the pulp passes between the refiner disks, the fibers are liberated and treated to a twisting and kinking action. However, experience has shown that dry refining using conventional disk refining equipment also produces unwanted fiber bundles and knots.
To offset this problem, it is proposed in U.S. Pat. No. 3,596,840, to Blomqvist et al, to introduce pulp into a disk refiner entrained in a carrier gas stream. In doing so, it is alleged that the number of fiber bundles or knots produced are reduced to about 10-15 percent. However, there is no information provided to show the amount of work done on the fibers in Blomqvist et al, and it is apparent that little fiber deformation or separation is accomplished since the pulp passes rapidly through the gap between the disks. Meanwhile, in U.S. Pat. No. 4,036,679, to Back et al, another dry refining process using a disk refiner is disclosed wherein the throughput rate is correlated with the rate of relative movement of the disks to produce convoluted, twisted, fiberized fibers which are nonfibrillated. In the latter patent, no carrier gases are used nor specifically required, and there is no information provided to show how many knots or fiber bundles are produced.
In contrast to the prior efforts noted above, the present invention is based on the belief that fiber bundles and knots are produced by an increase in the temperature of the refiner during operation, and because the individual fibers sometimes become lodged in the refiner where they receive excessive energy. Thus, the process and apparatus of the present invention treats cellulose pulp to produce fibers that are substantially permanently deformed, i.e., twisted, kinked and bent, but without producing unwanted fiber bundles and knots.